This invention relates to a class of broadband power amplifiers that can accurately produce output power without regard for normal power supply or load importance constraints. Analog amplifier designs historically have relied upon the principle of coupling a fixed power supply to a load impedance via elements exhibiting variable conductance. This is accomplished with a variety of devices, all of which perform the function of a valve. Accurate control of the output voltage or current can be achieved, but the amplifier must rely upon a voltage/current transformation device in order to exceed either the voltage or current limitations of the power supply when driving a non-ideal load impedance. The load impedance driven by many power amplifiers is often widely variable on a static and/or dynamic basis. In addition to manufacturing and environmental factors, load impedances in many applications are highly frequency-dependent. In that most existing power amplifiers control output voltage or current only, this limits output power accuracy to that of the driven load.
The advent of switching amplifier designs has improved efficiency. However, these designs have largely substituted pulsewidth modulation for variably-conductive coupling, and have therefore been subject to the same power supply constraints of their analog predecessors.
In applications where a limited or non-optimum power supply voltage or current is available to drive a fixed load impedance, existing amplifier designs require the use of voltage-transforming devices in order to provide the desired output power to the load impedance. These devices usually take the form of output transformers or voltage-transforming power supply sections which increase the voltage or current available to the output stage. Amplifiers designed with such output transformers can be plagued with frequency and phase distortion problems, as well as weight and size penalties. Transformer losses also limit the overall efficiency. Voltage-transforming power supplies may not impact size and weight appreciably, but significantly reduce system efficiency and reliability due to increased circuit complexity.
U.S. Pat. No. 4,180,782 uses an autotransformer in conjunction with a pair of output transistors to increase the output voltage in an analog power amplifier. However, the technique shown is limited to an output voltage twice that of the power supply, with efficiency no better than that typical of analog power amplifiers. There still exists a need for a class of amplifier that efficiently and accurately produces an output power without regard for normal power supply or load impedance constraints. The present invention provides such an amplifier by incorporating the impedance transformation function described above directly in the amplifier output stage, thereby improving system efficiency and reliability while reducing weight, size, and complexity. The approach provides capability of furnishing a controlled output power, in addition to more conventional voltage or current control. To accomplish these objectives, the present invention moves away from the traditional approach of variable coupling between the power supply and load impedance.